A reverse flow combustor for a gas turbine engine comprises an annular bulkhead or combustor dome in which is mounted a number of fuel nozzles. From the dome, inside and outside combustor liner walls extend to contain the combustion gases which reverse direction and exit the combustion zone via a large/outer exit duct and a small/inner exit duct towards the high and low pressure turbine zones. With all combustors, the space inside the combustor, or combustion volume, is designed to provide the desired combustion characteristics, while the space outside the combustor, between the combustor and surrounding engine case, is designed to permit the desired airflow around the combustor. However, the constraints of the engine configuration do not always permit both to be individually optimized, and consequently trade-offs are some time necessary. Nonetheless, there is a desire to improve the overall efficiency and performance of combustors, while ever reducing costs and weight.